Lithuanian investigators resumed their search of a crashed Russian fighter jet, one day after halting the probe over concerns about a radioactive substance found in the wreckage, according to chief investigator.

Investigators had not received a reply from Russia about what the substance could be, but they decided to resume sifting through the wreckage in the hopes of recovering a fourth air-to-air missile that remained missing, said Gen. Vitalijus Vaiksnoras, who is leading the investigation.

"We have decided to take maximum security measures and to resume digging," Vaiksnoras said.

Radiation experts deemed a radioactive substance found in the wreckage to be no threat to human health, but were still waiting for an official response from Russia as to the exact nature of the substance, Vaiksnoras said.

Investigators also repeated their plea to area residents to return scattered parts of the plane that they have been taking home as souvenirs.

The Su-27 fighter bomber crashed last week in Lithuania while traveling from St. Petersburg to the Russian enclave of Kaliningrad. The pilot, who ejected safely and was detained, is accused of violating Lithuanian airspace.

Lithuanian prosecutors say they will hand Troyanov back to Russia if the investigation finds the plane crashed due to technical failure.

If charged and convicted, he could face up to two years in prison, the AP reports.